At amzn a decision from atop was made that nobody would ever write in shared dynamo db tables. A team would own and provide APIs. That massively improved reliability and velocity.

The team boundary is very important. You can get away with shared DB for a long time if the same team handles all services that access it and have absolute tight control over them. If there are different teams in picture, however, the tight coupling is a source of problems and a bottleneck, beyond prototyping / idea validation, etc.

I don't need a decision from atop amazon to remind me how painful it would be to migrate a widely shared dynamo instance or god forbid change dax settings